Modelled effects of ambient UV radiation on a natural Antarctic marine microbial community

Ozone depletion over Antarctica has enhanced ultraviolet-B radiation (UVBR, 280 to 320 nm wavelength). We measured the effect of ambient solar UV radiation on the biomass and species composition of phytoplankton, protozoa, bacteria and dissolved organic carbon (DOC) in natural microbial assemblages from Antarctic coastal waters. Results were modelled to determine the features of the irradiance responsible for changes in the biomass of these microbial components and responses of individual phytoplankton taxa. Model results showed that changes in phytoplankton biomass were primarily due to dose rate, indicating that their UV-induced mortality resulted from the equilibrium between damage and repair. However, there was considerable variability between individual species in their response to dose and dose rate. Changes in protozoan biomass were mainly due to dose and were likely due to community-level, trophodynamic interactions. UV radiation did not measurably affect bacterial biomass, but resulted in increasing concentrations of DOC. We found a threshold of erythemal irradiance of 28 mW m-2, approximating peak noon-time irradiance at 3.6 m depth near the summer solstice in Antarctic coastal waters, below which no change in the community structure was observed, but above which phytoplankton mortality and protozoan biomass increased. Our results indicate that enhanced UVB radiation in Antarctic waters increases phytoplankton mortality and causes changes in the structure, function and composition of the microbial community that are likely to return more photoassimilated carbon to the atmosphere. © Inter-Research 2006.